Unusual Chantal Disorganized, but has 65 mph Winds

Unusual Tropical Storm Chantal has strengthened a bit more as it speeds west-northwestwards at 26 mph away from the the Lesser Antilles Islands. Sustained winds of 38 mph, gusting to 52 mph, were observed at Martinique at 10 am AST as the storm passed. However, an automated weather station at the airport measured sustained winds of 60 mph, gusting to 78 mph, according to an official with Meteo-France. The Associated Press reported that Chantal ripped the roofs off of several homes on neighboring Dominica. An Air Force hurricane hunter aircraft measured top winds at their 1,000' flight level of 89 mph at 12:55 pm AST. Top winds seen by the aircraft's SFMR instrument were about 65 mph, in a small area east of Chantal's center. The Hurricane Hunters have departed Chantal, and the next plane is due in the storm at 8 pm EDT. Chantal's winds are unusually high considering the storm's high central pressure of 1006 mb and disorganized appearance on satellite imagery. Chantal is fighting dry air associated with the Saharan Air Layer (SAL), as seen on water vapor satellite loops. This dry air is creating strong thunderstorm downdrafts that are robbing Chantal of moisture and energy. Visible satellite loops show the outflow boundaries of these thunderstorm downdrafts at the surface, spreading to the northwest of Chantal. Martinique Radar shows a large area of heavy rain that is not well-organized, lying mostly to the west of the Lesser Antilles Islands.

Figure 1. MODIS image of Chantal taken at approximately 1 pm EDT Tuesday, July 9, 2013. At the time, Chantal had top winds of 65 mph, but looked very disorganized, due to high wind shear and dry air. Dry air is creating strong thunderstorm downdrafts that are robbing Chantal of moisture and energy. Outflow boundaries from these downdrafts are spreading out to the northwest of Chantal, as seen on this satellite image. Image credit: NASA.

An small-scale easterly jet creating high shear in ChantalChantal is not very impressive on satellite images, with a modest amount of heavy thunderstorms that are not well-organized. Only a small amount of upper-level outflow is visible. The reason for Chantal's rather disorganized appearance can be found by looking at this morning's balloon sounding from Guadaloupe. This island was just northwest of the center of Chantal when the balloon was launched at 8 am EDT. The sounding showed typical easterly trade winds at the surface of 12 knots (14 mph.) However, the winds rose quickly aloft, with a jet of easterly winds of 35 - 53 knots between 800 - 600 mb (about 7,000 - 15,000'.) But, by the time the ballon hit 500 mb (18,000'), the winds had died down to 15 knots. A change of wind speed from 12 knots to 53 knots and back down to 15 knots from the surface to 500 mb is a tremendous amount of wind shear, which will make it very difficult for a tropical storm to keep the surface center aligned with the upper level center. The traditional measure of wind shear, the difference in wind between 200 mb and 850 mb, was 44 knots in this morning's Guadaloupe sounding, but was a much higher 56 knots from 200 mb to 700 mb. The powerful easterly wind jet was not apparent at any of the other balloon soundings this morning at adjacent islands (Barbados, Puerto Rico, Saint Martin), and demonstrates that there is a lot going on the atmosphere at small scales we cannot see which makes intensity forecasting of tropical cyclones very challenging. Thanks go to Jason Dunion of NOAA's Hurricane Research Division for pointing out this morning's interesting Guadaloupe sounding.

Forecast for ChantalChantal will have difficulty intensifying much more before hitting Hispaniola on Wednesday afternoon. In their 11 am EDT wind probability forecast, NHC gave Chantal a 29% chance of becoming a hurricane before hitting Hispaniola. Working against intensification will be the high wind shear from the strong mid-level easterly jet discussed above, plus the fast forward speed of the storm--tropical storms moving faster than 20 mph in the deep tropics usually have trouble intensifying. In addition, the Eastern Caribbean is an area where the trade winds accelerate, helping drive sinking air that discourages tropical storm intensification. Dry air will also slow down the intensification process. Interaction with the high mountains of Hispaniola and high wind shear may be able to destroy Chantal by Thursday. The 2 pm EDT Tuesday wind shear forecast from the SHIPS model calls for shear to rise to the high range, 20 - 35 knots, Tuesday night through Friday. On Saturday, when Chantal is expected to be in the Bahamas, moderate wind shear of 10 - 20 knots is predicted. If Chantal survives until Saturday, it will then have the opportunity to re-strengthen. The latest 12Z run of the European model (ECMWF) dissipates Chantal as it crosses Hispaniola. The 12Z run of the American GFS model has Chantal barely surviving.

Chantal's fast west-northwest forward speed of 26 mph will slow to 20 mph by Wednesday morning and then 10 mph by Thursday night, as the storm "feels" the presence of a trough of low pressure over the U.S. East Coast. This trough will steer Chantal to the northwest and then north-northwest across Hispaniola and into the Bahamas. The trough of low pressure pulling Chantal northwards is expected to lift out the the northeast over the weekend, leaving Chantal behind off the coast of Florida. High pressure will likely build in, potentially forcing Chantal westwards into the Florida or Southeast U.S. coast, with a possible Sunday landfall.

Quoting 932. ProgressivePulse:Chantal looks like she wants to make another run at a CDO this evening. Convection is expanding and deepening over the center currently. CIMMS still shows good convergence/divergence so she should be able to maintain. Shear is currently 20kts over the center.

At 00:47:30Z (last observation), the observation was 184 miles (296 km) to the S (174°) from San Juan, Puerto Rico (USA).

Product: Air Force High Density (HDOB) Message (URNT15 KNHC)Transmitted: 10th day of the month at 00:47ZDate: July 10, 2013Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 306)Storm Number: 03Storm Name: Chantal (flight in the North Atlantic basin)Mission Number: 3 Observation Number: 13

Quoting wunderkidcayman:well guys I said it and almost all of ya said no and call me wishcaster and westcaster so ha

You said what exactly? That Chantal would move more westerly, as she has done for her entire history, regardless of model persistence? Exactly how far west do you think she will go? There was never any doubt in my mnd that she would go further west than the models said but if you're implying that she will get to the Caymans, or even Jamaica, I would still call you a wishcaster.

My first impression was that the burst of convection did indeed represent the center. It may be that it is or will become the center after all is said and done. However, looking at the water vapor loop in rock mode made me think twice about that. Using that tool, the center seems to be where the NHC thinks it should be.

Time will tell.

If I had a nickel for every time someone said this was a storm unlike any other I'd be rich.

At 00:37:30Z (last observation), the observation was 159 miles (255 km) to the S (181°) from San Juan, Puerto Rico (USA).

Product: Air Force High Density (HDOB) Message (URNT15 KNHC)Transmitted: 10th day of the month at 00:37ZDate: July 10, 2013Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 306)Storm Number: 03Storm Name: Chantal (flight in the North Atlantic basin)Mission Number: 3 Observation Number: 12

Quoting 938. Stormchaser121:I think the models will continue to move further west. Thats the trend.

I'm inclined to agree if for no other reason than the most recent HH center fix is such a hard west turn from the advisory.

I guess two big things that are going to screw me up are the ULL has moved slower than I thought; I expected it to be a degree or so further west. Also, the very shallow convection this storm has generally maintained throughout it's existence would seem to undermine the mean steering layer product because it's pressure and convection thickness is not consistent with it's wind speeds. I'm not convinced steering is legit compared to an ordinary 60 to 65mph storm.

Quoting 932. ProgressivePulse:Chantal looks like she wants to make another run at a CDO this evening. Convection is expanding and deepening over the center currently. CIMMS still shows good convergence/divergence so she should be able to maintain. Shear is currently 20kts over the center.

Yep there it is just blew up in the last frame and westbound she goes.